Barking is rarely motivated by aggression, hence the saying "a barking dog will seldom bite". It's more like sounding an alarm, to alert the rest of the pack, including humans, to a potential danger, such as an intruder.
Unless the dogs are showing more obvious signs of aggression, such as snarling, lowering their bodies ready to leap, and so on, I'd say they're doing their job, ie guarding. OP's description of them walking alongside her to the boundary of the property sounds very much like they're trained fairly well, tbh, and that they make no attempt to chase.
I'm more concerned that they ignore the person who tries to call them. You refer to them as "the owner", OP: do you mean the owner of the dogs or of the property?
I've known people who own and train guard dogs, and every dog of theirs will come to them immediately they're called, but will ignore anyone else (including the handler's wife in two instances). If they're ignoring the person who has trained and handled them, they are clearly not under control and the person who has trained them should be present when they're running loose.
But the barking in itself does not in any way mean they're aggressive. A low growl, snarling, and the occasional bark accompanied by a lunge and/or a snap of the jaw (aka "air snap") are all strong indicators of aggression, but it doesn't sound like they're doing any of that.
I'd try and make friends with them, personally. Treats may not be the best idea (they may come to expect them, and make themselves more of a nuisance), but stopping and making a fuss of them, talking to them etc, would be good.
But equally there'd be no harm in reporting them. The dog warden would probably come out and assess if they were under adequate control or not.